The present disclosure refers to electricity, namely to basic electric elements, in particular to electric switches as well as to details of protective switches, which are triggered by means of combination of electrothermal and electromagnetic mechanism and are actuated by means of appropriate lever.
Embodiments of the present disclosure are directed to a switch, which could be integrated into each disposable electric circuit in order to protect it against short-circuit current and also against long-term excess current and/or voltage overloading, and moreover also against transient voltage, wherein such switch should have smallest possible dimensions and should be similarly like the other analogous switches integrated into a standardized casing adapted for mounting on a standardized ledge within common electric switchboards.
A switch for protecting against a current overloading or overloading due to excess current is described in DE 36 37 275 A1. Such switch comprises a core, which is inserted within a tubular casing and is furnished with a central passage, through which an axially movable actuating needle is inserted and protrudes therefrom towards the interrupting member i.e. a member, which is capable either to maintain the circuit established or to interrupt promptly and reliably the circuit, as soon as the actuating needle abuts it with the actuating end portion thereof. Said casing is wrapped with an electric conductive wire, which forms an electric winding i.e. a coil. In addition to said coil, an electromagnetic yoke is foreseen on the external surface of said casing, by which the coil is surrounded. The actuating needle is furnished with a widening, which is arranged apart from said core, so that the core is located on the one side of said widening, and the electromagnetic anchor is located on the opposite side thereof. A compression spring is inserted between said anchor and said core, so that the needle is surrounded with said spring. The needle extends throughout the core and also throughout the anchor, which is however axially feed on the needle. The actuating needle extends throughout said anchor and protrudes towards a bimetallic membrane, which is at room temperature deflected in a direction apart from the anchor and the core and is located apart from the corresponding end portion of the actuating needle, whilst by increasing of temperature it is deflected in the opposite direction i.e. towards the anchor and the core, and is rest onto that end portion of the actuating needle, which is protruding against it.
When the switch is integrated into each disposable electric circuit, the first end portion of the actuating needle is located apart from the bimetallic membrane, and the second end portion is located apart from the interrupting member, by which the electric circuit is established. Whenever the short-circuit current arises, the voltage is increased and appropriate magnetic field is generated within the coil, by which the anchor is shifted towards the core and the interrupting member by simultaneously abutting said widening on the needle, which is then pressed towards the interrupting member, by which, the circuit is interrupted.
Whenever a long-term excess current or voltage overloading arises, the heat is generated within the electric conductors, and the heat is transferred towards the bimetallic membrane, which is deflected in a direction towards the actuating needle in order to displace the last towards the interrupting member, by which the circuit is then interrupted. Meanwhile, the spring between the core and the anchor is compressed. As soon as the excess current is eliminated, the temperature in the area of the bimetallic membrane is decreasing, and the membrane is deflected back to its initial position, by which, the interrupting member allows establishing of the electric circuit and former operating thereof.
When considering the response of the anchor and/or the bimetallic membrane with respect to each irregularities within the electric circuit, such protective switch is able to react to irregularities or overloading in the context of excess current or voltage over loading within several ms. However, the so-called transient voltage, which occur e.g. in lightning, duration of each overvoltage stroke is usually several μs, so that effects and consequences thereof cannot be prevented by means of the previously mentioned switch, since neither the coil nor the bimetallic membrane is able to respond in such short time period.
Still further, a modular protecting apparatus is described in SI 21584A, which consists of a switch, which is triggered by means of the magnetic field, as well as of an overvoltage protecting circuit, which is serial connected to said switch in order to establish a common circuit. The switch is pretty similar to those described in DE 36 37 275 A1 and comprises a casing with a core and an anchor, as well as an actuating needle, which extends there-through and can be rest onto an interrupting member of the switch, by which the circuit is interrupted in the case of overloading. As soon as an excessive current or a voltage overloading takes place, a corresponding magnetic field is generated within the coil, which leads to displacing the anchor as well as the actuating needle towards the interrupting member. Said overvoltage protecting circuit comprises varistors, which are in parallel connected to each other and are intended to protect other components of the electric circuit from being overloaded in particular when transient voltage could arise. Such apparatus is then capable to protect the circuit against a short-circuit current and partially also against transient voltage, but the problem may arise, if the circuit needs to be protected against a voltage overloading, which would have some longer durability. In a practical circuit with nominal voltage 220 or 230 V this would mean a voltage overloading up to approx. 300 to 400 V in duration of approx. 1 s. In such case it could happen, that the magnetic field generated by the coil might be insufficient for triggering the actuating needle and interrupting the circuit, since due to such long-term voltage overloading also the varistors are thermally overloaded, which leads to destroying, even to explosion of varistors.